Quantum Theory Cannot Hurt You_ A Guide to the Universe - Marcus Chown [79]
INERTIAL FORCE A force we invent to explain a motion that is actually due to nothing more than inertia. A good example is centrifugal force. There is no such force flinging us outwards in a car rounding a sharp corner. We are simply continuing to move in a straight line because of our inertia, and the interior of the car, because it is moving along a curved path, intercepts us.
INFLATION, THEORY OF Idea that in the first split-second of its creation the Universe underwent a fantastically fast expansion. In a sense, inflation preceded the conventional Big Bang explosion. If the Big Bang is likened to the explosion of a grenade, inflation was like the explosion of an H-bomb. Inflation can solve some problems with the Big Bang theory such as the horizon problem.
INFRARED Type of invisible light that is given out by warm bodies.
INTERFERENCE The ability of two waves passing through each other to mingle, reinforcing where their peaks coincide and cancelling where the peaks of one coincide with the troughs of another.
INTERFERENCE PATTERN Pattern of light and dark stripes that appears on a screen illuminated by light from two sources. The pattern is due to the light from the two sources reinforcing at some places on the screen and cancelling at other places.
INTERSTELLAR MEDIUM The tenuous gas and dust floating between the stars. In the vicinity of the Sun this gas comprises about one hydrogen atom in every 3 cubic centimetres, making it a vacuum far better than anything achievable on Earth.
INTERSTELLAR SPACE The space between the stars.
ION An atom or molecule that has been stripped of one or more of its orbiting electrons and so has a net positive electrical charge.
ISOTOPE One possible form of an element. Isotopes are distinguishable by their differing masses. For instance, chlorine comes in two stable isotopes, with a mass of 35 and 37. The mass difference is due to a differing number of neutrons in their nuclei. For instance, chlorine-35 contains 18 neutrons and chlorine-37 contains 20 neutrons. (Both contain the same number of protons—17—since this determines the identity of an element.)
JOULE The standard scientific unit of energy. The energy of motion of a flying cricket ball is about 10 joules; the chemical energy provided by a single slice of bread is about 100,000 joules; and the electrical energy of a lightning discharge is about 10 billion joules.
LAMBDA POINT Temperature below which liquid helium begins to turn into a superfluid.
LASER Light source in which the gregarious nature of photons—bosons—comes to the fore. Specifically, the more photons there are passing through a material the greater the probability that other atoms will emit others with the same properties. The result is an avalanche of photons all travelling in lockstep.
LIGHT, CONSTANCY OF The peculiarity that in our Universe the speed of light in empty space is always the same, irrespective of the speed of the source of light or of anyone observing the light. This is one of two cornerstones of Einstein’s special theory of relativity, the other being the principle of relativity.
LIGHT, SPEED OF The cosmic speed limit—300,000 kilometres per second.
LIGHT BENDING See Gravitational Light Bending.
LIGHT-YEAR Convenient unit for expressing distances in the Universe. It is simply the distance that light travels in one year in a vacuum, which turns out to be 9.46 trillion kilometres.
LORENTZ CONTRACTION The contraction of a body moving relative to an “observer.” The observer sees the body shrink in the direction of its motion. The effect is noticeable only when the body is moving close to the speed of light with respect to the observer.
LUMINOSITY The total amount of light pumped into space each second by a celestial body such as a star.
MAGNETIC FIELD The field of force that surrounds a magnet.
MANY WORLDS The idea that quantum theory describes everything, not simply the microscopic world of